Gold plating electrolyte

ABSTRACT

A GOLD PLATING ELECTROLYTE CONTAINING AS ITS CONDUCTING AND BUFFERING COMPOUNDS ALKALI METAL OR AMMONIUM SULPHAMATE AND ALKALI METAL OR AMMONIUM PHOSPHATE. THE CRITICALITY OF THE RATIO OF 5:1 TO 2.3:1 SULPHAMATE TO PHOSPHATE IF MAINTAINED IN THE GOLD ELECTRLYTE BETWEEN THE PRESCRIBED LIMITS WILL GIVE GOLD DEPOSITS WITH EXCELLENT METAL DISTRIBUTION. TO MAINTAIN THE PRESCRIBED RATIO OF THE TWO RADICALS, THE GOLD IS REPLENISHED FROM AMMONIUM GOLD, CYANIDE, THEREBY MAINTAINING PH AND DENSITY AND CRITICAL RATIO OF SAID ELECTROLYTE.

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SUBSTRATE IN INCHES U.S. Cl. 204-43 G States Patent O v 3,776,821 GOLD PLATING ELECTROLYTE Kenneth D. Baker, Somerville, NJ assignor to EngelhardvMinerals & Chemicals Corporation Filed May 12, 1 972, Ser. No. 252,595 Int. Cl. C23b /28, 5/42 4 Claims ABSTRACT OF THE DISCLOSURE critical ratio of said electrolyte.

BACKGROUND OF INVENTION Gold has been electrodepositedfrom aqueous solutions n--inthe acid, neutral and alkaline medium, containing conducting and buffering salts such as phosphates, citrates, borates; carbonates and sulphamates of alkali metals or ammonium. These salts singularly or in combination have been used primarily as conducting and pH buffering materials, and when used in gold electrolytes have tended to give deposits with poor metal distribution. Also, there is the problem of replenishment of gold from such electrolytes. Normally, when gold is electroplated using insoluble anodes from a bath containing potassium gold cyanide, potassium ions accumulate in the solution, thereby increasing the alkalinity of such solution. To compensate for the increase in alkalinity either phosphoric acid or sulphamic acids would normally have been added and this would destroy the critical sulphamate to phosphate ratio according to the invention, and if added together within the correct molar ratio, the density of the electrolyte would increase, thereby placing a definite life span on such an electrolyte.

SUMMARY OF THE INVENTION The gold plating electrolyte of the invention contains gold as an alkali metal or ammonium cyanide complex, a sulphamate compound of alkali metal, ammonium or acid, a phosphate compound of alkali metal, ammonium or acid, arsenic trioxide as a brightener and sufficient alkali metal hydroxide or ammonium hydroxide to adjust the pH of the electrolyte between 5 and 8. The sulphamate and phosphate contents are maintained at the critical ratio of 5:1 to 2.3:1 sulphamate to phosphate.

DESCRIPTION OF THE DRAWING The single figure is a graphical representation illustrating the thickness of the gold distribution deposited by an electrolyte according to the invention in comparison with the thickness of the gold distribution deposited by an electrolyte outside the scope of the invention to obtain a minimum thickness of 90 microinches.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It has been determined according to the invention that when using a mixture of sulphamate and phosphate ions within a prescribed ratio of 5:1 to 2.321, respectively, in a gold plating electrolyte, a gold deposit is obtained where metal distribution is good in comparison with deposits from a conventional gold plating bath. It has been further determined according to the invention that when using ice ammonium gold cyanide as the gold source for replenishing the gold depleted during electrolysis, the ammonium ions do not accumulate either as the hydroxide or the cyanide, but are volatile and unstable, thereby maintaining the critical sulphamate to phosphate ratio according to the invention without shortening the life of the electrolyte by an increase in density or crystallization of breakdown products.

A gold electrolyte according to the invention contains:

Gold as an alkali metal or ammonium cyanide complex gms /ltr 2-30 sulphamate compound of alkali metal, ammonium or acid gms./ltr 20-100 Phosphate compound of alkali metal, ammonium or acid gms./ltr 5-45 Arsenic trioxide as a brightener mg./ltr 2-10 Sufiicient alkali metal hydroxide or ammonium hydroxide to adjust the pH between 5 to 8.

The ratio of sulphamate ions to phosphate ions being 5:1 to 2.3:1.

The following are examples of electrolytes according to the invention:

EXAMPLE 1 Gold as potassium gold cyanide gms./ltr 12 Sulphamic acid gms./ltr 53 Tripotassium phosphate (ratio of sulphamate to phosphate 5:1) gms./ltr 18.5 Arsenic trioxide mg./ltr 3.5 Sufficient potassium hydroxide to adjust pH to 6.5.

EXAMPLE 2 Gold as potassium gold cyanide gms./ltr 12 Ammonium sulphamate gms./ltr 74 Ammonium dihydrogen phosphate (ratio of sulphamate to phosphate 3.7:1) gms./ltr 25 Arsenic trioxide mg./ltr 3.5 Suflicient ammonium hydroxide to adjust pH to 6.5.

EXAMPLE 3 Gold as potassium gold cyanide gms./ltr 8.5 Gold as ammonium gold cyanide gms./ltr 4.0 Sulphamic acid gms./ltr 42.5 Tripotassium phosphate (ratio of sulphamate to phosphate 4:1) gms./ltr 15.75 Arsenic trioxide mg./ltr 3.5

Sufficient potassium hydroxide to adjust pH to 6.5.

following is an example of such an electrolyte:

EXAMPLE 4 Gold as potassium gold cyanide gms./ltr 12 Ammonium sulphamate gms./ltr 26 Ortho phosphoric acid (ratio of sulphamate to phosphate 1:1.12) gms./ltr 25 Arsenic trioxide --mg/ltr-.. 3.5

Suflicient potassium hydroxide to adjust pH to 6.5.

The above comparisons of plating thickness are illustrated in the figure where the graphical illustration shows the small extent of overplate required to deposit a minimum thickness of microinches from a solution (Exphosphate within the critical ratio of :1 to 2.3 :1 sulpha mate to phosphate as compared with the more uneconomical greater overplating required to deposit a minimum thickness of 90 microinches when a solution (Example 4) outside the scope of the invention is employed.

In accordance with the invention the sulphamate compounds contemplated are preferably those selected from the group consisting of sulphamic acid, ammonium sulphamate, potassium sulphamate and sodium sulphamate, and the preferred phosphate compounds are those selected from the group consisting of tripotassium phosphate, dibasic potassium phosphate, monobasic potassium phosphate, trisodium phosphate, dibasic sodium phosphate, monobasic sodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate and ortho phosphoric acid.

Various modifications of the electrolyte of the invention are contemplated within the scope of the appended claims.

What is claimed is:

1. An aqueous gold electroplating electrolyte comprismg (a) 2-30 grams/liter of gold as an alkali metal or ammonium cyanide complex,

(b) 20-100 grams/liter of a sulphamate compound of alkali metal, ammonium or acid,

(c) 545 grams/liter of a phosphate compound of alkali metal, ammonium or acid,

(d) the ratio of the sulphamate ions to phosphate ions being 5:1 to 23:1,

(e) 2-10 mg./ liter of arsenic trioxide as a brightener,

(f) and sufiicient alkali metal hydroxide or ammonium hydroxide to adjust the pH to 5-8.

2. An aqueous gold plating electrolyte according to claim 1, wherein the gold content is selected from the group consisting of potassium gold cyanide, sodium gold cyanide and ammonium gold cyanide.

3. An aqueous gold plating electrolyte according to claim 1, wherein the sulphamate compound is selected from the group consisting of sulphamic acid, ammonium sulphamate, potassium sulphamate and sodium sulphamate.

4. An aqueous gold plating electrolyte according to claim 1, wherein the phosphate compound is selected from the group consisting of tripotassium phosphate, dibasic potassium phosphate, monobasic potassium phosphate, trisodium phosphate, dibasic sodium phosphate, monobasic sodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate and ortho phosphoric acid.

References Cited UNITED STATES PATENTS 3,617,452 11/1971 Keith et a1 204-46 3,423,295 1/ 1969 Greenspan 20443 GERALD L. KAPLAN, Primary Examiner US. Cl. X.R. 20446 

